1. Field of the Invention
The present invention relates generally to a cellular system including a relay, and in particular, to an effective method for transmitting multicast data in a Worldwide Interoperability for Microwave Access/Wireless Broadband (WiMax/WiBro) relay system.
2. Description of the Related Art
In order to enhance signal power and suppress interference when transmitting multicast data in a WiMax/WiBro relay system, it is often necessary for both a Base Station (BS) and several Relay Stations (RSs) participating in the multicast service to simultaneously transmit data with the same coding/modulation. In such transmission schemes, the interference from adjacent RSs or BSs usually becomes useful signals. For a User Equipment (UE), signals from different transmitters can be regarded as multi-path signals. As long as the difference between the signals is within the guard interval of an Orthogonal Frequency Division Multiplexing (OFDM) symbol, they can be correctly received as useful signals.
When multicast data is transmitted in a conventional WiMax/WiBro relay system, extreme differences exist between channels because the distances between the receivers and the RSs or the BSs are different. In order to guarantee that the receiver on a channel having poor quality can correctly decode data with some acceptable probability, a lower-order modulation scheme and more powerful coding are usually applied to multicast data. However, this is a severe waste of resources for UEs having good channel conditions.
In a Terrestrial Digital Video Broadcast (DVB-T) scheme, a method called hierarchical modulation is proposed, in which low-order modulation is embedded in high-order modulation for transmission. For instance, QPSK is embedded in 64 QAM or 16 QAM. The hierarchical modulation method bears close relation to the video transmission in DVB-T. A piece of high-definition video can be usually divided into a basic layer and multiple enhancement layers. The basic layer can offer video with standard definition. The video cannot be decoded with only the enhancement layer, and video streams of higher definition can be provided only when the basic layer is used together. Therefore, the basic layer is the stream with high priority and usually adopts lower-order modulation plus strong coding scheme to guarantee that it can be correctly received by UEs; the enhancement layers are the streams with low priority and adopt high-order modulation and weak coding scheme for transmission. With the hierarchical modulation method, the basic layer with high priority is embedded in the enhancement layers levels with low priority for the purpose of transmission. UEs having good channel conditions can decode both the basic layer and the enhancement layers simultaneously in order to obtain high-definition video, and UEs having poor channel conditions can decode the basic layer to obtain video with basic definition.
In the hierarchical modulation method, enhancement layer signals with high-order modulation interfere with basic layer signals with low-order modulation in the constellation. Consequently, an adjustable separation factor α is proposed in DVB-T, which means that, in the constellation, at least the distance of a least constellation points should exist between every two of constellation points of low-order modulation. Therefore, increasing α can reduce the interference caused by high-order modulated signals to low-order modulated signals. However, increasing α also reduces the least constellation point distance. Consequently, higher SINR is required to distinguish two different constellation points of high-order modulation. Actually, the separation factor α can regulate the interference between high-order modulated signals and low-order modulated signals. The greater α is, the more interference low-order modulated signals inflict on high-order modulated signals; the smaller α is, the more interference high-order modulated signals cause to low-order modulated signals.
The hierarchical modulation solution is suitable for video data transmission in broadcast services, but it is not suitable for application in either WiMax or WiBro system for a number of reasons.
First, for the hierarchical modulation solution, it is necessary to modify the receiver greatly, especially when the constellation needs to be adjusted according to the separation factor α. All the current amended versions of WiMax and WiBro system emphasize that the receiver needs to be backward compatible.
Second, the high-order modulation part in the hierarchical modulation solution can only be used by UEs in good channel conditions in signal receiving. In non-video services, if UEs in good channel conditions has no data to receive, resource of such part is still wasted.
Third, with the hierarchical modulation solution, although interference between high-order modulated signals and low-order modulated signals can be regulated by using the separation factor to adjust the constellation, because the two types of signals are transmitted via the same constellation, there is no way to regulate transmitting power for different signals, which leads to the loss of power control, an effective approach for interference suppression.